222 



Mr. E. H. Griffiths. 

 Table VII. 



[Jan. 1' 



merits gave the " total heat," not the value of L, the results should be 

 different after subtraction of the " total heat " of the water, for when 

 subtracting we assumed the identity of the two units. 



Dieterici adopted as the basis of his measurements the mass of 

 mercury drawn into a Bunsen's calorimeter by the heat evolved from 

 1 gram of H 2 when cooling from 100 to JO .* Now, according to 

 Eegnault,f the ratio of the " mean thermal unit " to the "thermal 

 unit at C. " is as 1-005 : 1. 



If we assume Rowland's, or Bartoli and Stracciati's, determination 

 of the changes below 15 (my own have not extended below that 

 temperature), we should get 



mean thermal unit 



1-005 . , . 101-1 



approximately = - 



thermal unit at 15 0'994 



and thus Dieterici's value of L at 0, if expressed in terms of thermal 

 unit at 15, would become 603'3. 



Again, according to Eegnault we ought to subtract 100'5 from 

 636-60, and thus get L at 100 = 536'1 . 



The only reasonable explanation of this difficulty appears to me to- 

 be that the value of the " mean thermal unit " is practically the same 

 as the value of the "thermal unit at 15 C." 



I see nothing impossible in this supposition. As shown in Paper L, 

 there is sufficient evidence that at low temperatures the capacity for 

 heat of water decreases with rise of temperature. Rowland found a 

 minimum indicated near 34. If, therefore, the capacity for heat 

 increases gradually above some such temperature, but more rapidly 

 near 100, it is quite conceivable that the "mean thermal unit" 

 should closely approximate to the "thermal unit at 15 C." Our 

 only experimental evidence to the contrary is that given by Regnault 

 in his paper " De la Chaleur Specinque." We know that his con- 

 clusions at low temperatures are incorrect, and I do not see that 

 those at higher temperatures have greater value, for his methods of 

 observation and experiment were in this case unaltered. The matter, 



* ' Wied. Ann.,' vol. 37, 1889, p. 499. 



t " De la Chaleur Specifique." ' Acad. des Sciences,' tome xxi. 



